1. Field of the Invention
This invention relates to resonance testing of head gimbal assemblies (HGAs) to be used in disk drive suspensions, and, more particularly, to apparatus and method that use piezo actuators in shear mode (d15) rather than the conventional electromagnetic/electrodynamic shakers, or non-shear mode (d33) piezo stacks to provide a movement impetus to the HGA that is useful in resonance testing and in other dynamics sensing. Shear-mode piezo actuators are compact and provide linear, repeatable movements at frequencies up to several tens of kilohertz, importantly with minimal out-of-plane movement. As a further advantage the present testing devices can be used for such separate functions as windage testing, FRF testing, and micro-actuator stroke testing.
2. Description of the Related Art
As recording track densities increase and servo bandwidths also increase, there is an urgent need to test HGA resonance at high frequencies. In the prior art, mostly electromagnetic/electrodynamic shakers have been used in resonance testers. But testers using electromagnetic/electrodynamic shakers have been found to interfere with the obtaining of accurate results by interacting with HGA vibration modes. An electromagnetic/electrodynamic shaker, for example, with large off-axis motion produces in the HGA unwanted out-of-plane modes that cross couple with the in-plane HGA modes.
Most HGA resonance testers, then, suffer from unpredictable dynamics as a result of off-axis compliance problems; these problems are exacerbated at higher frequencies. Attempts at mechanically minimizing the off-axis movements with bearings and guide structures have helped, but mechanical expedients introduce further problems, such as friction and the need for continual lubrication, that affect repeatability over time.
The use of d33 mode piezo stack transducers in place of the electromagnetic/electrodynamic shakers has been suggested. Although these piezo transducers exhibit relatively repeatable performance, their out-of-plane movements are huge when mounted in the suggested cantilevered arrangement. At some frequencies, the out-of-plane movement (displacement) of these d33 piezo stacks even exceed the desired in-plane movements